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Tannis was a Time Lord from Gallifrey.
Larna mentioned Tannis to Ulysses, Saldaamir, Penelope Gate, and Marnal in a list of entities that would threaten Gallifrey in their future (her relative past). She said it was very important that Gallifrey should survive Tannis's attack to ensure that the world would be destroyed at its proper time. (PROSE: The Gallifrey Chronicles)